Supplemental inflatable restraint (SIR) systems for automotive vehicles generally employ a firing circuit having a squib for causing inflation of an air bag, a deployment circuit having an accelerometer sensitive to vehicle motion, especially deceleration, and a microprocessor monitoring the accelerometer output for evaluating the severity of a crash to determine whether to deploy the air bag. The vehicle battery or ignition system voltage empowers the deployment circuit and the firing circuit. The deployment circuit and the firing circuit are to a large extent carried out by a microprocessor on an integrated circuit chip, but an external harness leads to a squib at the site of each air bag.
It is conceivable that some portion of the harness might become damaged and short to ground or even to the vehicle battery voltage. To forestall the possibility that such inadvertent electrical connection to the firing circuit might cause deployment of the air bag or disable the system, it is desirable to monitor the system to detect any such event. It is already known to diagnose a short of the firing circuit by feeding a small test current through the squib and through a biasing resistor to ground. The resulting bias voltage will depend on the current and if there are no shorts to the firing circuit the resulting test voltage will be at a prescribed value. To assure this, the test current is carefully controlled. Where, for example, the current is provided by a current source on an integrated circuit, the circuit must be trimmed during manufacture to assure the correct current output. The test voltage is sampled by an A/D converter and fed to the microprocessor where it is monitored to detect a low value indicative of shorting to ground or a high value indicative of shorting to supply voltage.
The firing circuit includes a pair of FETs, one on each side of the squib. To deploy the air bag, the squib is fired by turning on both FETs. Thus to assure readiness to fire it is desirable to occasionally test the FETs (and their drive circuits) for operability. In prior systems the FETs were tested by commanding on a single FET at a time, and monitoring for the voltage on the deployment loop to go high (for a high side FET test) or low (for a low side FET test). This was accomplished by monitoring the low or high side of the deployment loop with an amplifier and then disabling the FET after a brief duration (several hundreds of microseconds). A microprocessor would be responsible for monitoring the low side of the squib, first checking to see that the squib was not shorted to battery or ground, and then commanding on the FET to be diagnosed. After a few hundred microseconds of delay, the amplifier would be checked, and the FET then disabled. With that arrangement it is conceivable that a short to ground (or battery) could occur immediately before or during testing the high side (or low side) FET. This has the possibility of causing an inadvertent deployment or degradation of the squib.
It has been proposed in U.S. patent application Ser. No. 08/651,073, filed May 22, 1996, now Pat. No. 5,666,065, entitled "FAST ACTING FET TEST CIRCUIT FOR SIR DIAGNOSTICS" by Ravas et al and assigned to an assignee of this application, to apply a bias voltage to a firing circuit, monitoring the firing circuit voltage to sense either a short or a FET turn-on, and then rapidly terminating FET conduction. The monitoring step comprises comparing the circuit voltage to upper and lower thresholds. While that circuit is very fast and efficient, it has a potential problem of not detecting a short to another circuit which applies a voltage within the range prescribed by the thresholds.